Bertrand Lambolez

Molecular and Physiological Diversity of Cortical Nonpyramidal Cells

The physiological and molecular features of nonpyramidal cells were investigated in acute slices of sensory-motor cortex using whole-cell recordings combined with single-cell RT-PCR to detect simultaneously the mRNAs of three calcium binding proteins (calbindin D28k, parvalbumin, and calretinin) and four neuropeptides (neuropeptide Y, vasoactive intestinal polypeptide, somatostatin, and cholecystokinin). In the 97 neurons analyzed, all expressed mRNAs of at least one calcium binding protein, and the majority (n = 73) contained mRNAs of at least one neuropeptide. Three groups of nonpyramidal cells were defined according to their firing pattern. (1) Fast spiking cells (n = 34) displayed tonic discharges of fast action potentials with no accommodation. They expressed parvalbumin (n = 30) and/or calbindin (n = 19) mRNAs, and half of them also contained transcripts of at least one of the four neuropeptides. (2) Regular spiking nonpyramidal cells (n = 48) displayed a firing behavior characterized by a marked accommodation and presented a large diversity of expression patterns of the seven biochemical markers. (3) Finally, a small population of vertically oriented bipolar cells, termed irregular spiking cells (n = 15), fired bursts of action potentials at an irregular frequency. They consistently co-expressed calretinin and vasoactive intestinal polypeptide. Additional investigations of these cells showed that they also co-expressed glutamic acid decarboxylase and choline acetyl transferase. Our results indicate that neocortical nonpyramidal neurons display a large diversity in their firing properties and biochemical patterns of co-expression and that both characteristics could be correlated to define discrete subpopulations.

AMPA receptor subunits expressed by single purkinje cells

Several subunits of the glutamate receptor of the AMPA subtype have been cloned recently. These subunits, named GluR1, GluR2, GluR3, and GluR4, exist as two splicing variants (flip and flop). We have determined the subset of AMPA receptor subunits expressed by single cerebellar Purkinje cells in culture. This was achieved by combining whole-cell patch-clamp recordings and a molecular analysis, based on the polymerase chain reaction, of the messenger RNAs harvested into the patch pipette at the end of each recording. We found that each single cell expresses the messenger RNAs encoding the following five subunits: the flip and flop versions of GluR1 and GluR2 as well as GluR3flip, GluR2 being the most abundant. In addition, GluR3flop and GluR4flip were scarcely expressed in half of these neurons, and GluR4flop was never detected.

Subunit composition at the single-cell level explains functional properties of a glutamate-gated channel

The diversity of known glutamate-gated channels has been markedly increased by the discovery of multiple subunits and their spliced and edited variants. These subunits can potentially form different oligomeric complexes with diverging properties. A crucial question is therefore to determine the actual subunit composition of naturally occurring glutamate receptors. We have coupled patch-clamp recordings and reverse transcription followed by PCR amplification to correlate the presence of mRNAs for each subunit and the functional properties of native glutamate receptors at the single-cell level. In a homogeneous population of functionally identified hippocampal neurons (type II) in culture bearing a glutamate receptor of the AMPA subtype with a high calcium permeability, we found that, among the multiple subunits, only two, the flop forms of GluR1 and GluR4, were expressed. In particular, GluR2 was never detected. This composition explains the uncommon properties of AMPA receptors in type II neurons.

Selective Excitation of Subtypes of Neocortical Interneurons by Nicotinic Receptors

The cellular mechanisms by which neuronal nicotinic cholinergic receptors influence many aspects of physiology and pathology in the neocortex remain primarily unknown. Whole-cell recordings and single-cell reverse transcription (RT)-PCR were combined to analyze the effect of nicotinic receptor agonists on different types of neurons in acute slices of rat neocortex. Nicotinic receptor agonists had no effect on pyramidal neurons and on most types of interneurons, including parvalbumin-expressing fast spiking interneurons and somatostatin-expressing interneurons, but selectively excited a subpopulation of interneurons coexpressing the neuropeptides vasoactive intestinal peptide (VIP) and cholecystokinin. This excitation persisted in the presence of glutamate, GABA, and muscarinic receptor antagonists and in the presence of tetrodotoxin and low extracellular calcium, suggesting that the depolarization was mediated through the direct activation of postsynaptic nicotinic receptors. The responses were blocked by the nicotinic receptor antagonists dihydro-β-erythroidine and mecamylamine and persisted in the presence of the α7 selective nicotinic receptor antagonist methyllycaconitine, suggesting that the involved nicotinic receptors lacked the α7 subunit. Single-cell RT-PCR analysis indicated that the majority of the interneurons that responded to nicotinic stimulation coexpressed the α4, α5, and β2 nicotinic receptor subunits. Therefore, these results provide a role for non-α7 nicotinic receptors in the selective excitation of a subpopulation of neocortical interneurons. Because the neocortical interneurons expressing VIP have been proposed previously to regulate regional cortical blood flow and metabolism, these results also provide a cellular basis for the neuronal regulation of cortical blood flow mediated by acetylcholine.

The endogenous somnogen adenosine excites a subset of sleep-promoting neurons via A2A receptors in the ventrolateral preoptic nucleus.

Recent research has shown that neurons in the ventrolateral preoptic nucleus are crucial for sleep by inhibiting wake-promoting systems, but the process that triggers their activation at sleep onset remains to be established. Since evidence indicates that sleep induced by adenosine, an endogenous sleep-promoting substance, requires activation of brain A(2A) receptors, we examined the hypothesis that adenosine could activate ventrolateral preoptic nucleus sleep neurons via A(2A) adenosine receptors in rat brain slices. Following on from our initial in vitro identification of these neurons as uniformly inhibited by noradrenaline and acetylcholine arousal transmitters, we established that the ventrolateral preoptic nucleus comprises two intermingled subtypes of sleep neurons, differing in their firing responses to serotonin, inducing either an inhibition (Type-1 cells) or an excitation (Type-2 cells). Since both cell types contained galanin and expressed glutamic acid decarboxylase-65/67 mRNAs, they potentially correspond to the sleep promoting neurons inhibiting arousal systems. Our pharmacological investigations using A(1) and A(2A) adenosine receptors agonists and antagonists further revealed that only Type-2 neurons were excited by adenosine via a postsynaptic activation of A(2A) adenosine receptors. Hence, the present study is the first demonstration of a direct activation of the sleep neurons by adenosine. Our results further support the cellular and functional heterogeneity of the sleep neurons, which could enable their differential contribution to the regulation of sleep. Adenosine and serotonin progressively accumulate during arousal. We propose that Type-2 neurons, which respond to these homeostatic signals by increasing their firing are involved in sleep induction. In contrast, Type-1 neurons would likely play a role in the consolidation of sleep.

Kainate Receptor Subunits Expressed in Single Cultured Hippocampal Neurons: Molecular and Functional Variants by RNA Editing

To determine the kainate receptor subunits that are found in native kainate receptors, we have applied a multiplex PCR of cDNAs reverse transcribed from mRNA harvested from single cultured hippocampal neurons after electrophysiological recording. We found that all the cells showing rapidly desensitizing currents in response to kainate express the GluR6 subunit mRNA, and that some of them also express the GluR5 subunit mRNA. No GluR7, KA-1, or KA-2 subunit mRNAs were detected. Analysis of the editing sites of the GluR6 mRNA demonstrated that the three editing sites present in these subunits are edited to a different extent. Predominant expression of the unedited variant (Q) was observed, but edited and unedited variants may coexist in the same cell. In addition, we show that the Q/R site from the GluR6 subunit controls functional properties of native kainate receptors.

Properties of bipolar VIPergic interneurons and their excitation by pyramidal neurons in the rat neocortex

In the rat neocortex, a subset of GABAergic interneurons express the neuropeptide vasoactive intestinal peptide (VIP). Previously, we demonstrated that a population of VIPergic interneurons could be accurately identified by their irregular spiking (IS) pattern and their bipolar morphology. IS interneurons were studied in neocortical slices from 16–22‐day‐old rats using whole‐cell recordings, intracellular labelling and single‐cell RT‐PCR. In response to a depolarizing pulse, IS interneurons typically discharged a burst of action potentials followed by spikes emitted at an irregular frequency. Several seconds of depolarization, micromolar concentrations of 4‐aminopyridine, and nanomolar concentrations of either dendrotoxin I or K converted this irregular pattern to a sustained discharge, suggesting the involvement of an ID‐like K+ current. The main glutamate receptor subunits detected in IS cells were GluR1 flop and GluR2 flop, GluR5 and GluR6, and NR2B and NR2D for the α‐amino‐3‐hydroxyl‐5‐methyl‐4‐isoxazolepropionic acid (AMPA), kainate and N‐methyl‐d‐aspartic acid (NMDA) subtypes, respectively. Paired whole‐cell patch‐clamp recordings indicated that pyramidal neurons provide intracortical glutamatergic inputs onto IS interneurons. Most connections had high probabilities of response and exhibited frequency‐dependent paired pulse depression. Comparison of the amplitude distribution of paired responses suggested that most of these connections consisted of multiple functional release sites. Finally, two discrete subpopulations of IS cells could be identified based on the duration of the initial burst of action potentials and the differential expression of calretinin and choline acetyltransferase.

Activity-dependent regulation of N-methyl-D-aspartate receptor subunit expression in rat cerebellar granule cells.

The glutamate receptor channels of the N‐methyl‐d‐aspartate (NMDA) subtype are composed of different subunits named NR1 and NR2A‐D. These subunits can combine in different oligomers with diverging properties and their expression is developmentally regulated. We have used rat cerebellar slice cultures to test the involvement of bioelectrical activity and synaptic transmission in the changes in NR2A‐C expression observed in developing granule cells. A correlation between the functional properties of the NMDA receptors and expression of the NR2A‐C mRNAs was obtained in single granule cells by coupling patch‐clamp recording and reverse transcription followed by polymerase chain reaction. Granule cells grown under standard culture conditions expressed mainly NR2A mRNA when examined after 15–40 days in vitro. Consistent with this observation, their responses to NMDA were only weakly reduced by 3 μM ifenprodil, a non‐competitive antagonist which discriminates between NR2A and NR2B subunits in expression systems. In cerebellar cultures chronically exposed to tetrodotoxin to eliminate spontaneous electrical activity, granule cells maintained a predominant expression of NR2B subunits and their responses to NMDA were largely inhibited by 3 μM ifenprodil. These results provide evidence that the expression of the NR2A and B subunits is regulated through an activity‐dependent mechanism leading to the formation of NMDA receptors with different pharmacological properties. Finally, the NR2C subunit, abundantly expressed in vivo by adult granule cells, was only rarely detected in slice cultures, even when excitatory synapses were formed between granule cells and fibres originating from co‐cultured brainstem explants. These data suggest that the induction of NR2C expression observed in vivo requires an additional factor(s) that remains to be identified.

Cellular locus of the nitric oxide-synthase involved in cerebellar long-term depression induced by high external potassium concentration

The cellular location of the NO-synthase involved in long-term depression (LTD) of parallel fiber (PF)-mediated EPSCs induced by raising the external potassium (K) concentration has been investigated by using both whole-cell patch-clamp recordings (WCR) of Purkinje cells (PCs) in thin slices in vitro, and reverse transcription followed by polymerase chain reaction (PCR) applied to mRNAs harvested from these single PCs during WCR. In all tested cells in the control group, a large LTD of PF-mediated EPSCs was induced by perfusing the slices for 3 min with a high (30 mM) K perfusing medium. In a second group of cells for which the protein kinase C (PKC) inhibitor peptide 19-36 was added to the intrapipette solution at a concentration of 10 microM, the LTD following complete wash out of the high K solution was significantly less prominent than in the control group. Very similar results were also obtained when 30 microM NG-methyl-L-arginine (L-NMMA) was added to the perfusing medium. In contrast, when both the PKC inhibitor peptide 19-36 and L-NMMA were added to the intrapipette solution at a concentration of 10 and 30 microM respectively, no LTD was revealed following wash out of the high K solution. Finally, the PCR amplification of mRNAs harvested from these single PCs during WCR, as well as from granule cells from the same slices, confirms that mRNAs encoding the NO-synthase are expressed by granule cells, whereas they are not detected in PCs.

Dynamics of protein kinase A signaling at the membrane, in the cytosol, and in the nucleus of neurons in mouse brain slices

The cAMP-dependent protein kinase A (PKA) plays a ubiquitous role in the regulation of neuronal activity, but the dynamics of its activation have been difficult to investigate. We used the genetically encoded fluorescent probe AKAR2 to record PKA activation in the cytosol and the nucleus of neurons in mouse brain slice preparations, whereas the potassium current underlying the slow afterhyperpolarization potential (sAHP) in thalamic intralaminar neurons was used to monitor PKA activation at the membrane. Adenylyl cyclase was stimulated either directly using forskolin or via activation of 5-HT7 receptors. Both stimulations produced a maximal effect on sAHP, whereas in the cytosol, the amplitude of the 5-HT7 receptor-mediated response was half of that after direct adenylyl cyclase stimulation with forskolin. 5-HT7-mediated PKA responses were obtained in 30 s at the membrane, in 2.5 min in the cytosol, and in 13 min in the nucleus. Our results show in morphologically intact mammalian neurons the potential physiological relevance of PKA signal integration at the subcellular level: neuromodulators produce fast and powerful effects on membrane excitability, consistent with a highly efficient functional coupling between adenylyl cyclases, PKA, and target channels. Phosphorylation in the cytosol is slower and of graded amplitude, showing a differential integration of the PKA signal between the membrane and the cytosol. The nucleus integrates these cytosolic signals over periods of tens of minutes, consistent with passive diffusion of the free catalytic subunit of PKA into the nucleus, eventually resulting in a graded modulation of gene expression.